A variety of technologies for preventing copying of a banknote, a credit card or a high-priced article is known in the art. Among these technologies, there is a method in which a hard-to-copy label is affixed to an article to be protected from copying and in which the copying or non-copying of the article is indirectly determined by determining the copying or non-copying of the label. For example, a label printed with a special ink or a label printed with a hologram is affixed to a high-priced article, a credit card, a banknote or a passport. The authenticity of an article such as a credit card or the like is indirectly determined by determining the authenticity of a label affixed thereto. Since the label is designed to be broken when separated from the article, the authenticity of the label-affixed article can be indirectly guaranteed through the use of the label. The label used for this purpose is referred to as a product authentication label.
The product authentication label can be used in case where it is more difficult to copy a label than to copy a product. In reality, most of the labels developed thus far can be copied. Thus, a demand has existed for a hard-to-copy security label. In order for a security label for the indirect determination of copying or non-copying of an article to be commercially used, the security label should comply with the following conditions.
1. A label should have substantially unique physical features which are distinguishable from other labels. This is because, if a label has unique physical features, it is possible to assure the uniqueness of an article to which the label is affixed.
2. It is required that the physical features of the label can be extracted with ease and the extracted physical features can be easily converted to a digital identification code which permits determination of authenticity. The similarity of the converted digital identification code should be assured such that the authenticity of a label can be determined even if the extraction (reading) conditions of the physical features are changed within a specified range depending on the device.
3. In a label manufacturing factory, it should be technically difficult to manufacture a label having the same physical features such that a manufacturer gives up an attempt to copy the same label as the previously manufactured one. This reliably assures the uniqueness of a label-affixed article.
4. It should be difficult and extremely costly to copy a label. The cost required in copying a label should overwhelm the benefit obtained by copying the label. Copying should be difficult and costly such that one gives up an attempt to copy a label.
5. A method for reading physical features of a label should be trustworthy and a device for reading physical features of a label should be inexpensive. The cost required in manufacturing a label and determining the copying or non-copying of a label should be quite low such that the benefit obtained by affixing a label is greater than the loss occurring when a security label is not used.
GB 2 304 077 A discloses a label that satisfies some of the aforementioned conditions. The label (security device) disclosed in GB 2 304 077 A includes a plurality of reflective particles randomly distributed in a substrate made of a synthetic resin or a coating layer. The reflective particles are metallic flakes or reflection-coated granules which have a plurality of reflective surfaces. It is preferred that the reflective particles are not spheroids. Thus, the light incident on the label in a specified direction is reflected in a predetermined direction depending on the distribution position and posture of the reflective particles. A reflection pattern or a reflection signal is received using a camera or an optical sensor. The reflective particles are put into a bulk-type synthetic resin and are stirred and randomly mixed. The mixture is injection-molded into a card. For that reason, the distribution position and posture of the reflective particles are randomly decided. The probability that labels identical in the position and posture of particles are manufactured is extremely low. Accordingly, each of the labels thus manufactured is unique in terms of the probability. The pattern of the light reflected from the label including the reflective particles having a unique position and posture is also unique. The unique pattern of the reflected light is used as a label identification code.
However, the label disclosed in GB 2 304 077 A fails to satisfy the second condition required in a label. The label disclosed in GB 2 304 077 A includes particles each having a plurality of reflection surfaces and exhibits a physical feature that the light irradiated in a specific direction is reflected or not reflected depending on the position and posture of the particles. However, the label is very sensitive to the physical feature reading conditions (e.g., the direction or intensity of incident light and the position of a camera or an optical sensor for receiving reflected light). Even if the condition is changed a little bit, the label exhibits a completely different reflection feature. That is to say, even in the same label, the light reflection pattern taken by a camera or the signal obtained through an optical sensor shows a great change depending on the slight change in the light irradiation angle or the slight change in the position of the camera or the optical sensor. Particularly, in case where the label is affixed to a credit card or an article, different reflection patterns are obtained even in the same label if the position of a light source or a camera of a reader device installed at different locations is changed only a little bit. This makes it impossible to determine the authenticity of the label. The reflection patterns of all the security labels using particles having a plurality of angled surfaces are extremely sensitively changed depending on the change in the position of a light source or a camera. Thus, the security labels are hard to assure the reliability of authenticity determination and are not commercially available thus far.
GB 2 324 065 A discloses a label in which a plurality of visually distinguishable beads is randomly distributed in a matrix made of a synthetic resin. The information on the positions of beads existing in the label is read by an optical reader. The information is coded and stored in a database. The label affixed to an article is scanned by an optical reader, thereby reading the positions of beads. The copying or non-copying of the article is verified by comparing the positions of beads with the stored codes. The beads reflect the light coming from all directions. This provides an advantage in that the two-dimensional positions of the beads within the matrix can be accurately grasped using the image of the label taken by the camera.
However, the label disclosed in GB 2 324 065 A suffers from a problem in that the label can be copied with ease. Even if the beads are three-dimensionally and randomly distributed within the matrix, it is possible to easily copy the label in case where only the two-dimensional positions (the X and Y coordinates on a plane) of the beads are used as identification codes. By imaging a genuine label with a camera, the arrangement positions of the beads can be identified through the use of an image processing technique. The label can be perfectly copied by two-dimensionally arranging the beads through the use of a precision machine and then fixing the beads with a synthetic resin. Alternatively, a copied label can be easily manufactured by imaging a genuine label with a camera and then outputting the imaged two-dimensional pattern with a precision printer. In case of authenticating a label only with a machine, a reader installed in the machine reads the same image as the genuine label from a printed label but cannot distinguish the printed label from the genuine label. For example, a machine such as an ATM (Automatic Teller Machine) or the like provided with a camera as a reader cannot distinguish a credit card having a printed label from a credit card having a genuine label.
US 2007/0170257 A1 discloses a method for determining whether particles having a light reflection property are three-dimensionally distributed in a label. The particles are three-dimensionally and randomly distributed in a carrier layer. The particles are in the form of glass beads or balls or in the form of discs. The method disclosed in US 2007/0170257 A1 is directed to a method for distinguishing a label copied by imaging a label containing particles such as glass beads or the like and printing the imaged label from a genuine label having three-dimensionally distributed glass beads.
In the method disclosed in US 2007/0170257 A1, the position of a camera is fixed. Three label images are obtained using light sources installed at different positions. Based on an image obtained by combining the three images, it is determined whether the particles are three-dimensionally distributed. The light sources installed at different positions are turned on to irradiate light on a label. Three label images are taken by a camera. The reflection angle of the light irradiated on the glass beads varies depending on the positions of the light sources. Thus, the three images are taken as if the positions of the particles are slightly moved. In case where the three images taken as if the positions of the particles are slightly moved are combined, a geometric artifact appears in the combined image. If the geometric artifact exists in the combined image, it is determined that the label is a genuine label. If the geometric artifact does not exist in the combined image, the particles are not three-dimensionally distributed. It is therefore determined that the label is copied label. This method is applicable because the glass beads have a property of reflecting the light irradiated in all directions.
The method disclosed in US 2007/0170257 A1 is capable of distinguishing a printed label from a genuine label having a three-dimensional structure. However, the method cannot distinguish a label copied so as to have a three-dimensional structure from a genuine label. For example, in case where material capable of causing light interference is coated on a photograph obtained by printing an image taken with a camera such that the angle of the reflected light varies depending on the positions of light sources, there is a possibility that the method cannot distinguish the copied label from a genuine label.
Furthermore, in case where a three-dimensional label is copied by reading two-dimensional coordinates of particles from a image taken with a camera, disposing particles at two-dimensional coordinate positions through the use of a precision machine and fixing the particles with a synthetic resin, there is a problem in that it is impossible to distinguish the copied label from a genuine label.